Brassinolide (structure (1), FIG. 1) is a steroidal natural product that was first isolated in 1979 (Grove et at.). It was shown to affect the growth of some plant species at doses as low as 1 nanogram per individual plant (Grove et al.). Subsequently, other brassinosteroids have been discovered in diverse plant natural sources (Cutler et al., Adam et al., Fujioka et al.) or synthesized (Cutler et al.; Back, 1995). It has been shown that brassinolide increases cell division and cell elongation in plants (Grove et al., Cutler et al., Adam et al., Mandava) and enhances the biosynthesis of proteins (Cutler et al., Mandava), among other physiological effects. Field trials conducted with brassinolide and certain other brassinosteroids, such as 24-epibrassinolide (2) and 28-homobrassinolide (3), have shown that applications of as little as 10-100 mg per hectare result in significant improvements in the yields of crops such as wheat, rice, potatoes, barley, and others (Cutler et al., Mandava). Improved stress resistance to drought, temperature extremes and salinity has also been noted in plants treated with brassinosteroids.
Brassinolide has been generally considered to be the most active naturally-occurring brassinosteroid. One synthetic analogue, 25-homobrassinolide (4), has been reported to be "slightly more active" than brassinolide in the rice leaf lamina inclination bioassay (Mori et al., 1988) and to promote growth of mung beans (Mori et al., 1987). Other analogs for which plant growth promoting activity has been reported include the 24-epi and 28-homo analogs of brassinolide, noted above, as well as the related keto compound, castasterone (5) (Hayashi et al.). Most of these analogs are reported to be either comparable in activity or somewhat less active than the parent compound (1).